1,4,7,10-tetraazacyclododecane-1,4-diacetic acid

ABSTRACT

1,4,7,10-tetraazacyclododecane-1,4-diacetic acid of formula (I):                    
     as well as its chelated complex salts with bi-valent metal ions having atomic numbers from 20 to 31, 39, 42, 43, 44, 49, or from 57 to 83, as well as their salts with anions of physiologically acceptable organic acid selected from acetate, succinate, citrate, fumarate, maleate, oxalate, or with anions of inorganic acids selected from halo acids ions. The compounds are intermediates for preparing 1,4,7,10-tetraaza-cyclododecane chelating agents.

The present invention relates to the novel compound, acid1,4,7,10-tetraazacyclododecane-1,4-diacetic of formula (I), itscomplexes with paramagnetic metal ions and physiologically compatiblesalts thereof, as well as to the preparation thereof and the use thereoffor the preparation of chelating agents.

The compound of formula (I) is a novel chelating agent for bi-trivalentmetal ions and is also an important intermediate for the synthesis of1,4,7,10-tetraazacyclododecane derivatives chelating agents,functionalized at the 1-and 4-positions with the acetic residue.

The compound of formula (I) is the starting material for the synthesisof multidentate derivatives which are capable of complexing differentmetals, some of which have applications in the biomedical field, such asgadolinium complexes of said derivatives, which are used in diagnosticas contrast agents for the magnetic resonance technique (MagneticResonance Imaging, MRI).

Such complexes have been described, inter alia, in EP 325762.

Therefore, the object of the present invention is the compound offormula (I):

as well as its chelated complex salts with bi-trivalent metal ionshaving atomic number from 20 to 31, 39, 42, 43, 44, 49, or from 57 to83, as well as their salts with anions of physiologically acceptableorganic acids, selected from, for example, acetate, succinate, citrate,fumarate, maleate, oxalate, or with anions of inorganic acids such ashalo acids ions, specificallychlorides, bromides, iodides.

Metal ions suitable for preparing chelated complex salts with the novelchelating agent (I) are mainly bivalent or trivalent ions of theelements having atomic number variable from 20 to 31, 39, 42, 43, 44,49, or from 57 to 83; particularly preferred being Fe⁽²⁺⁾, Fe⁽³⁺⁾,Cu⁽²⁺⁾, Cr⁽³⁺⁾, Gd⁽³⁺⁾, Eu⁽³⁺⁾, Dy⁽³⁺⁾, La⁽³⁺⁾, Yb⁽³⁺⁾ or Mn⁽²⁺⁾ alsoradioisotopes such as ⁵¹Cr, ⁶⁷Ga, ⁶⁸Ga, ¹¹¹In, ^(99m)Tc, ¹⁴⁰La, ¹⁷⁵Yb,¹⁵³Sm, ¹⁶⁶Ho, ⁹⁰Y, ¹⁴⁹Pm, ¹⁷⁷ _(Lu,) ⁴⁷Sc, ¹⁴²Pr, ¹⁵⁹Gd, ²¹²Bi.

The novel compounds of the present invention have a good tolerability;moreover, their water solubility and the limited osmolality of theirsolutions are a further advantageous characteristic which makes themparticularly suitable for use in nuclear magnetic resonance.

Both soluble and less soluble compounds are useful for the oral andenteral administrations and, therefore, for the imaging of thegastronitestinal (GI) tract.

As far as the parenteral administration is concerned, the compounds arepreferably formulated as a sterile aqueous solution or suspension, whosepH can range for instance from 6.0 to 8.5.

These aqueous solutions or suspensions can be administered inconcentrations ranging from 0.002 to 1.0 mol.

These formulations can be lyophilized and supplied as they are forreconstitution before use.

For the GI use or for the injection in the body cavities, such agentscan be formulated as a solution or suspension containing suitableadditives which can control viscosity.

In the oral administration they can be formulated according topreparation methods commonly used in the pharmaceutical practice,possibly also as coated formulations, in order to get additionalprotection from the stomach acid pH, by preventing the release of thechelated metal ion occurring in particular at pH which are typical ofgastric juices.

Other excipients, for instance sweeteners and/or flavouring agents, canalso be added according to known techniques of pharmaceuticalformulation.

In the diagnostic field, the chelated complex salts of this inventioncan be used as contrast agents, while as radiopharmaceuticals in nuclearmedicine, they are useful both in the diagnostic and therapeutic sector.

In this case, however, the metal ion which is chelated is aradioisotope, for instance ⁵¹Cr, ⁶⁷Ga, ⁶⁸Ga, ¹¹¹In, ^(99m)Tc, ¹⁴⁰La,¹⁷⁵Yb, ¹⁵³Sm, ¹⁶⁶Ho, ⁹⁰Y, ¹⁴⁹Pm, ¹⁷⁷Lu, ⁴⁷Sc, ¹⁴²Pr, ¹⁵⁹Gd and ²¹²Bi.

The compounds object of the present invention can optionally bechemically conjugated with suitable macromolecules or included insuitable carriers.

Preferred anions of inorganic acids suitable for salifying the chelatedcomplex salts of the invention comprise, in particular, halo acids ionssuch as chlorides, bromides, iodides or other ions, such as sulfate.

Preferred anions of organic acids suitable for the above aim comprisethose of the acids conventionally used in pharmaceutical technique forthe salification of basic substances, such as acetate, succinate,citrate, fumarate, maleate.

Preferred amino acid anions comprise, for example, those of taurine,glycine, lysine, arginine or ornithine, or of aspartic and glutamicacids.

The chelated complex salts of compound (I) with the metal ions definedabove are prepared according to procedures known in literature, byreacting compound (I) with the oxide or the halide of the selected metalion.

More specifically, the reaction is carried out in water or in a suitablewater-alcohol mixture, and the temperature can range from 25° C. to 100°C., preferably from 40° C. to 80° C.

The choice of the metal ion and of any neutralizing ion is closelyrelated to the intended use of the complex to be prepared.

The preparation of the novel compound, manganese chelated complex of1,4,7,10-tetraazacyclododecane-1,4-diacetic acid, which is in theneutral form and therefore does not require the formation of aphysiologically compatible salt, is described in the Experimentalsection.

The compound of formula (I) was surprisingly prepared starting fromoctahydro-2a,4a,6a,8a-tetraazapentalen[1,6-cd]pentalene (CAS RN54364-78-2) of formula (II), and from1,2,3,4,6,7,8,9-octahydro-5H-4a,7,9a-triaza-2a-azoniacycloocta[cd]pentalene chloride of formula (IV), both obtainable according to knownmethods from 1,4,7,10-tetraazacyclododecane (III) (commonly namedCyclen), according to the following Scheme 1, through the formation ofthe novel compound of formula (V), 1,4,7,10-tetraazabicyclo[8.2.1]tridecane-13-on-4,7-diacetic acid, which is also an object of theinvention:

The compound of formula (II) quickly and reversibly dissociates in waterto give the compound of formula (IV), the respective preparations ofsaid compounds being already disclosed in U.S. Pat. No. 3,932,451 and ina paper (Richman et Simmons, Tetrahedron, 30, 1769, 1974), for the useof both of them in photography.

The compound of formula (II) is obtainable by benzene extraction fromthe alkalinized solution of compound (IV), which can easily be preparedin quantitative yields from 1,4,7,10-tetraazacyclododecane (commercialproduct, commonly named Cyclen) of formula (III), by reaction with ethylorthocarbonate and an equivalent of hydrochloric acid in ethanol, asshown in the following Scheme:

The compound of formula (IV) is in prototropic equilibrium at roomtemperature with compound (II) and with the dicationic compound (VI).This equilibrium is also of conformational type, thus causing themagnetic equivalence of protons of compound (II).

U.S. Pat. No. 3,932,451 discloses the preparation of these compoundsstarting from 2,3,5,6-tetraidro-1H-imidazo[1,2a]-imidazole, as shown inthe following Scheme:

The compound of formula (II) quickly reconverts to compound (VII) byreaction with ethyl chloroformate, thus giving further structuralevidences.

From such a behaviour of the compounds of formula (II) and (IV),documented in literature, no uses of these compounds as usefulintermediates for the synthesis of 1,4-disubstituted1,4,7,10-tetraazacyclododecane derivatives could be expected.

It is a further object of the invention the process for the preparationof the novel compound of formula (I), starting from known compounds offormula (II) or (IV), through the formation of the novel compound offormula (V), comprising the following steps represented in Scheme 2:

in which

step a) is the alkylation reaction in basic conditions with an aceticacid reactive derivative, XCH₂COOH, in which X is a halogen;

step b) is the basic hydrolysis under pressure and at a temperatureranging from 150-220° C.

The alkylation conditions in step a) are conventional ones: the reactiontemperature can range from 30 to 70° C.; the reaction time usuallyranges from 10 to 25 hours; the basic pH ranges from 10 to 12 and isobtained by addition of a inorganic base, preferably sodium or potassiumhydroxide; the amount of alkylating agent is stoichiometric or in aslight excess (up to 50%).

Preferred conditions in the presence of at least 2 mols of BrCH₂COOH asalkylating agent per mol of starting product are the following:temperature 45° C.; reaction time 21 hours; pH 11.5.

Compound of formula (V) is purified by elution of the acidified finalsolution on a polystyrene adsorbing resin, such as XAD-1600, to removethe salts, and subsequent recovery of the desired product.

Compound (V), whose structure was confirmed by spectroscopic analysis(¹H-NMR, ¹³C-NMR, IR and MS), shows a high stability under hydrolyticconventional conditions.

It has surprisingly been found that the basic hydrolysis of thiscompound at high temperature and under pressure causes the loss of thecarbonyl bridge, while keeping the acetic residues intact, therebyyielding simply and efficiently compound of formula (I).

The basic hydrolysis is carried out in aqueous medium at basic pH byaddition of an amount of inorganic base, as defined above, correspondingto 4-7 mols per mol of compound (V) at temperatures of 150-220° C.;pressure depending of course by the selected temperature to carry outthe reaction according to ideal gas law; the reaction time ranging from15 to 30 hours.

Preferred conditions are as follows: temperature 195° C. and pressure 10bars; 5 mols of NaOH per mol of compound (V); reaction time 22 hours.

The compound of formula (I) is in its turn a useful starting product forthe synthesis of the compounds of general formula (VIII),

which are useful chelating agents of paramagnetic metal ions, for thepreparation of contrast agents for magnetic resonance imaging, asdescribed in EP 325762.

It is therefore an object of the invention the process for thepreparation of compounds (VIII), starting from compound (I), byalkylation, according to known methods, with an excess of alkylatingagent R—CH(X)—COY of formula (IX), optionally followed by hydrolysis ofthe ester groups present, as shown in the following Scheme 3:

in which

R is a hydrogen atom, a straight or branched or cyclic C₁-C₆ alkylgroup, unsubstituted or substituted by 1 to 10 oxygen atoms, or a C₁-C₂₀alkyl group, optionally interrupted by a phenylene, phenylenoxy orphenylenedioxy group, in its turn substituted by a straight or branchedC₁-C₆ alkyl group, unsubstituted or substituted by 1 to 7 hydroxy groupsor 1 to 3 C₁-C₇ groups; the aromatic group can be unsubstituted orsubstituted by alkoxy groups or by halogens, carboxy, carbamoyl,alkoxycarbonyl, sulfamoyl, hydroxyalkyl, amino, acylamino, acyl,hydroxyacyl groups;

X is a halogen or a sulfonic acid reactive residue,

Y is a group —OH or —OR₁, wherein R₁ is a straight or branched C₁-C₄alkyl group; when Y is —OR₁, the ester groups are subjected to ahydrolysis step, according to known methods, to obtain compounds (VIII).

The alkylating agents of formula (IX) corresponding to compound offormula (X), R—CH(X)—COOH, in which X is bromine or chlorine, arepreferred; the alkylating agents of formula (XI), XCH₂COOH, in which Ris a hydrogen atom and X is bromine or chlorine, being most preferred.

In the other cases the alkylating agent of formula (IX) can be selectedfrom compounds that are commercially available or the preparation ofwhich has already been described in literature (see for example WO93/24469 or EP 325762), or from those still to synthesize, using forexample known methods for the preparation of suitable precursors (forexample, in case of acyl chlorides α-halogen derivatives, see: Harpp etal., J. Org. Chem., 40, 3420, 1975), and subsequent transformation intothe desired product.

Preferably R can be selected from the group consisting of: H or astraight or branched alkyl group, such as a methyl, ethyl, propyl,isopropyl, butyl, isobutyl group, in its turn substituted by hydroxygroups or interrupted by oxygen atoms, as defined above.

When an aromatic group is present in R, particularly preferred are thephenyl, benzyl, phenylmethoxymethyl groups.

Particularly preferred are 3-(phenylmethoxy)propanoic acid reactivederivatives, such as 2-bromo-3-(phenylmethoxy)propanoic acid, thepreparation of which is described in Grossman et al., Chem. Ber., 91,538, 1958, and 2-chloro-3-(phenylmethoxy)propanoic acid (CAS RN124628-32-6), prepared analogously to the brominated derivative.

On the other hand, the group R₁ is preferably selected from: methyl,ethyl, isopropyl, butyl, tert-butyl.

The reactive group X can be selected, by way of example, from the groupconsisting of halogens (Cl, Br, I), or it is a mesylate (MeSO₂O−),benzenesulfonyloxy (PhSO₂O⁻), nitrobenzenesulfonyloxy (p-NO₂PhSO₂O⁻),tosylate (TsO⁻) or triflate (CF₃SO₃ ⁻) group.

Particularly preferred are the compounds in which X is a halogen, abromide or a chloride being most preferred.

The alkylation of compound (I), when Y is the hydroxy group, canconveniently be performed with secondary carboxylic acids reactivederivatives, such as 2-bromopropionic acid, in aqueous alkalinesolution, at temperatures from 25 to 55° C.

Particularly preferred are the alkylating agents of general formula (X),in which Y is a hydroxyl group, corresponding to bromoacetic acid(commercially available product), 2-bromopropionic acid (commerciallyavailable product), 2-bromobutyric acid (commercially availableproduct).

On the other hand, when the alkylation reaction is carried out with anester derivative of compound (IX), the reaction solvent can suitably beselected from dipolar aprotic solvents, in particular fromdimethylformamide (DMF), dimethylacetamide (DMA), dimethylsulfoxide(DMSO), acetonitrile (CH₃CN) and N-methylpyrrolidone, and the reactionis carried out in the presence of an organic base, preferably a tertiaryaliphatic amine selected from triethylamine (TEA), diisopropylethylamineand tributylamine.

In this case it can be convenient to transform also the acid groups(—COOH) present in compound (I) into the ester groups (—COOR₁), in orderto promote the alkylation reaction, depending on the reactivity of thealkylating agent itself.

The reaction temperature will range, in this case, from 0 to 80° C.,depending on the reactivity of the selected alkylating agent.

In this case, the alkylation reaction will be followed by basichydrolysis of the resulting diester, in conventional conditions, toobtain the desired compound of formula (VIII).

By way of example of the huge potentialities provided by this syntheticroute, the synthesis of the novel compound,α,α′-bis(methyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaceticacid:

as well as that ofα,α′-bis((phenylmethoxy)methyl)-4,7,10-tetraazacyclododecane-1,4,7,10-tetraaceticacid:

are reported in the Experimental section, in which catalytichydrogenation, as described in example 6 of the cited Patent, leads toα,α′-bis(hydroxymethyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaceticacid.

In the following, some preparation examples according to the method ofthe present invention are reported.

EXPERIMENTAL SECTION Example 1 Synthesis ofoctahydro-2a,4a,6a,8a-tetraazapentalen[1,6-cd]pentalene

200 g of 1,4,7,10-tetraazacyclododecane (1.16 mol) are dissolved in 3 Lof toluene. Water is removed by azeotropical distillation to a residualcontent of 0.25% (Karl Fisher). The volume of the solution is restoredby adding a toluene amount corresponding to that distilled off. Thesolution is added with 100 mL of propionic acid (1,37 mol), at atemperature of 80° C., then heated to 90° C. adding 304 mL of ethylorthocarbonate (1.40 mol). The solution is reacted for 22 h at 90° C.,then cooled and concentrated by evaporation under vacuum, to obtain anoily residue, corresponding to 320 g of the desired product.

GC assay: 93.5% (area %)

¹H-NMR, ¹³C-NMR, IR and MS spectra are consistent with the structure.

Example 2 Synthesis of1,4,7,10-tetraazabicyclo[8.2.1]tridecan-13-on-4,7-diacetic acid

The residue obtained according to the procedure described in example 1,is dissolved in 2 L of deionized water. 2N NaOH to pH 12 and 452 g of an80% w/w bromoacetic acid aqueous solution (2,60 mol). The mixture isheated to 45° C. and reacted at pH 11.5 (keeping this pH by gradualadditions of 2N NaOH) for 21 h.

The solution is cooled, acidified with 34% HCl w/w to pH 1.1 andconcentrated by evaporation under vacuum, to a weight of 2.7 kg.

The solution is percolated on 15 L of adsorbing resin XAD-1600 elutingwith water. The fractions containing the useful product and free fromsalts are collected, combined and concentrated to dryness.

266 g (0.84 mol) of the desired product are thereby obtained.

Yield: 75% (calculated on the amount of 1,4,7,10-tetraazacyclododecaneof Example 1)

¹H-NMR, ¹³C-NMR, IR and MS spectra are consistent with the structure.

Example 3 Synthesis of 1,4,7,10-tetraazacyclododecane-1,4-diacetic acid

260 g of 1,4,7,10-tetraazabicyclo[8.2.1]tridecan-13-one (0.827 mol) aredissolved in 1 L of deionized water. 0.56 kg of 30% NaOH w/w are addedto the solution which is placed in autoclave. The solution is left at195° C. for 24 h, cooled, acidified to pH 4 with 34% HCl and filteredthrough paper.

The filtrate is percolated on 12 L of resin Relite 3ASFB, washing firstwith water, then eluting the product with 7 L of 1M HCl, 8 L of 0.5M HCland 20 L of water, in this order. The fractions containing the productare combined and concentrated under vacuum to a weight of 3.5 kg. Theresulting solution is percolated on a column containing 10 L ofpolyvinylpyridine resin, eluting with water. The fractions containingthe desired product are combined and concentrated to obtain 290 g of anoily residue, which is redissolved in 1.4 L of methanol. The solution isagain concentrated to dryness, to obtain 205 g of a solid that isrecrystallized from the methanol/acetone mixture.

147 g of the desired product (0.81 mol) are obtained.

Yield: 61%

¹H-NMR ¹³C-NMR and MS spectra are consistent with the structure.

Example 4 Synthesis of1,2,3,4,6,7,8,9-octahydro-5H-4a,7,9a-triaza-2a-azonia-cycloocta[cd]pentalene(CAS RN 54364-76-0) chloride

The procedure described by Richman and Simmons (Tetrahedron, 30. 1769,1974) is followed.

50 g of 1,4,7,10-tetraazacyclododecane (0.29 mol) yield 59.5 g (0.27mol) of the desired product (95% yield).

Example 5 Synthesis of1,4,7,10-tetraazabicyclo[8.2.1]tridecan-13on-4,7-diacetic acid startingfrom1,2,3,4,6,7,8,9-octahydro-5H-4a,7,9a-triaza-2a-azoniacycloocta[cd]pentalenechloride

The product obtained according to Example 4 is dissolved in 500 g ofdeionized water. 2N NaOH is added to adjust to pH 12 and 106 g (0.60 molof an 80% w/w bromoacetic acid aqueous solution are added The mixture isreacted as already described in Example 2.

63 g of the desired product (0.19 mol) are obtained.

Yield: 69%

¹H-NMR, ¹³C-NMR, IR and MS spectra are consistent with the structure.

Example 6 Synthesis ofα,α′-bis(methyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaceticacid

28 g (0.095 mol) of 1,4,7,10-tetraazacyclododecane-1,4-diacetic acid,prepared according to Example 3, are dissolved in 250 mL of deionizedwater. The solution is heated to 100° C. for 8 h, cooled to roomtemperature, diluted with 45 g of water and slowly added with a solutionprepared dissolving 45.89 g (0.300 mol) of 2-bromopropionic acid in 40mL of water. The mixture is reacted at a temperature of 45° C. for 25 h,keeping pH at 10.5-11 by addition of 2N NaOH. The mixture is cooled toroom temperature and acidified to pH 2 with conc. hydrochloric acid.After 1 h the precipitated solid is filtered, washing with deionizedwater. The crude product is redissolved in 600 mL of polyvinylpyridineresin (PVP), eluting thoroughly with water. The useful fractions arecombined, concentrated to dryness under vacuum and dried in static drierat 50° C. under vacuum, to obtain 35.3 g (0.080 mol) of the desiredproduct.

Yield: 84%

¹H-NMR, ¹³C-NMR, IR and MS spectra are consistent with the structure.

Example 7 Synthesis ofα4,α7-bis[(phenylmethoxy)methyl]-1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetraaceticacid

1,4,7,10-Tetraazacyclododecane-1,4-diacetic acid is reacted with2-bromo-3-(phenylmethoxy)propanoic acid methyl ester or with2-trifluoromethanesulfonate-2-(phenylmethoxy)propanoic acid methyl esterin DMF and in the presence of TEA. The methyl ester is hydrolysed toobtain the desired product.

¹H-NMR, ¹³C-NMR, IR and MS spectra are consistent with the structure.

Example 8

Synthesis ofα1,α4-bis(hydroxymethyl)1,4,7,10-tetraazacyclodode-cane-1,4,7,10-tetraaceticacid

The product obtained in Example 7 is subjected to catalytichydrogenation in water and in the presence of 5% Pd/C, to obtain, afterthe necessary hydrogen has been used, the desired product.

¹H-NMR, ¹³C-NMR, IR and MS spectra are consistent with the structure.

Example 9 Preparation of 1,4,7,10-tetraazacyclododecane-1,4-diaceticacid manganese chelated complex

8.66 g of compound (I) prepared as described in Example 3 (30 mmol) aredissolved in 30 mL of water. pH is adjusted to 6.8 by addition of asolution of 1-deoxy-1-(methylamino)-D-glucitol (0.3 mL; 0.3 mmol), thenMnCl₂ (30 mL; 30 mmol) is added in two hours, keeping pH at 6.8 byaddition of 1-deoxy-1-(methylamino)-D-glucitol (28.2 mL; 28,2 mL). After24 h the solution is filtered through a Millipore® (HA-0.22 μm) filter,nanofiltered, evaporated and the residue is dried on P₂O₅ to give thedesired product (5,0 g; 14,65 mmol). The permeate containing the desiredproduct is concentrated to 50 mL and the resulting solid (megluminechloride) is filtered off. The solution is evaporated to a residue,which is crystallized from MeOH (50 mL) to yield 3 g of a second crop ofthe desired product (8,0 mmol).

Yield: 76%

¹H-NMR, ¹³C-NMR, IR and MS spectra are consistent with the structure.

What is claimed is:
 1. A process for the preparation of compound (I),starting from octahydro-2a,4a,6a,8a-tetraazapentalen[1,6-cd]pentalene offormula (II), or from1,2,3,4,6,7,8,9-octahydro-5H-4a,7,9a-triaza-2a-azoniacycloocta[cd]pentalenechloride of formula (IV), by formation of1,4,7,10-tetraazabicyclo[8.1.1]tridecan-13-on-4,7-diacetic acid offormula (V), said process comprising the following steps represented inScheme 2:

in which step a) is an alkylation reaction in basic conditions with thealkylating agent XCH_(2═)COOH, in which X is a halogen; step b) is abasic hydrolysis under pressure and at a temperature ranging from150-220° C.
 2. A process as claimed in claim 1 in which in step a) thereaction temperature ranges from 30 to 70° C.; pH ranges from 10 to 12and the amount of alkylating agent is stoichiometric or up to 50%excess.
 3. A process as claimed in claim 2, in which the alkylatingagent used is BrCH₂COOH in a molar ratio of at least 2 mols per mol ofstarting product and in step a) temperature is 45° C. and pH is 11.5. 4.A process as claimed in claim 1, in which in step b) the basichydrolysis is carried out in aqueous medium at basic pH by the additionof an amount of a base corresponding to 4-7 mols per mol of compound(V), at temperatures of 150-220° C.
 5. A process as claimed in claim 4,in which temperature is 195° C. and pressure is 10 bar; and the base isadded in an amount of 5 mols of NaOH per mol of compound (V). 6.1,4,7,10-Tetraazabicyclo[8.2.1]tridecan-13-on-4,7-diacetic acid as anintermediate in the process according to claim
 1. 7. A process for thepreparation of a compound of formula (VIII), starting from compound (I),by alkylation with an excess of alkylating agent R—CH(X)—COY of formula(IX), optionally followed by reaction hydrolysis of the ester groupspresent, as shown in the following Scheme 3: Scheme 3:

in which R is a hydrogen atom, a straight or branched or cyclic C₁-C₆alkyl group, unsubstituted or substituted or the alkyl group isinterrupted by 1 to 10 oxygen atoms, or a C₁-C₂₀ alkyl group, optionallyinterrupted by a phenylene, phenylenoxy or phenylenedioxy group, in turnsubstituted by a straight or branched C₁-C₆ alkyl group unsubstituted orsubstituted by 1 to 7 hydroxy groups the phenylene group can beunsubstituted or substituted by alkoxy groups or by halogens, carboxy,carbamoyl, alkoxycarbonyl, sulfamoyl, hydroxyalkyl, amino, acylamino,acyl, or hydroxyacyl groups; X is a halogen or a sulfonic acid reactiveresidue, Y is a group —OH or —OR₁, wherein R₁ is a straight or branchedC₁-C₄ alkyl group; when Y is —OR₁, the ester groups are subjected to ahydrolysis step, to obtain compounds (VIII).
 8. A process as claimed inclaim 7, in which the alkylating agent of formula (IX) is a compoundR—CH(X)—COOH of formula (X), and X is bromine or chlorine.
 9. A processas claimed in claim 8, in which the alkylating agent of formula (X) is acompound XCH₂COOH of formula (XI), in which R is a hydrogen atom and Xis bromine or chlorine.
 10. A process according to claim 7, in which Ris H or a straight or branched alkyl group in turn substituted byhydroxy groups or interrupted by oxygen atoms, as defined above; thephenylene group in R being selected from phenyl, benzyl,phenylmethoxymethyl; R₁ is selected from the group consisting of methyl,ethyl, isopropyl, butyl, and tert-butyl; X is selected from the groupconsisting of halogen a mesylate, benzenesulfonyloxy,nitrobenzenesulfonyloxy, tosylate and a triflate group.
 11. A process asclaimed in claim 10, in which the alkylating agent is2-bromo-3-(phenylmethoxy)propanoic acid or2-chloro-3-(phenylmethoxy)propanoic acid.
 12. A process as claimed inclaim 9, in which the alkylating agent of formula (XI) is selected fromthe group consisting of bromoacetic acid, 2-bromopropionic acid and2-bromobutyric acid.
 13. A process as claimed in claim 12 whereinα,α′-bis(methyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaceticacid is prepared.
 14. A process as claimed in claim 11 whereinα,α′-bis[(phenylmethoxy)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaceticacid is prepared.